(a) Field of the Invention
The present invention relates to a MOS FET (metal oxide semiconductor field effect transistor) driving circuit. More specifically, the present invention relates to a MOS FET with a main FET and a sense FET, and a driving circuit of the same.
(b) Description of the Related Art
In general, a current is allowed to flow through a MOS FET when an electrical field is applied to a gate of the MOS FET to form a channel between the drain and the source of the MOS FET. The MOS FET can be switched on/off by controlling the intensity of the electric field applied to the gate of the MOS FET.
To switch the MOS FET on/off, a MOS FET driving circuit for controlling the voltage applied to the gate of the MOS FET and a sense FET for detecting the intensity of the current flowing to the MOS FET are required.
FIG. 1 is a circuit diagram of a conventional MOS FET driving circuit.
As shown in FIG. 1, the conventional MOS FET driving circuit senses load current IL through a current sensing resistor RS and thereby controls the drive of the MOS FET. This driving circuit is relatively simplified in construction such that upon a rapid increase in the load current IL, under excessive conditions, the voltage applied to the current sensing resistor RS is raised and the voltage Vgs between the gate and the source of the MOS FET is reduced. The effect is an increase in resistance Rds between the drain and the source of the switched on MOS FET, which limits the load current and prevents a breakdown of the MOS FET.
FIG. 2 is a circuit diagram of another conventional MOS FET driving circuit.
As shown in FIG. 2, in another example of the conventional MOS FET driving circuit, the drain of a main FET is connected to that of a sense FET and the gate of the main FET is connected to that of the sense FET. The source of the sense FET is connected to current sensing resistor RS. The main FET and the sense FET are contained in a single chip and are similar to each other in characteristics. But, a current ratio is set such that almost all the current flows towards the main FET to reduce energy consumption at the current sensing resistor.
However, the conventional MOS FET driving circuit shown in FIG. 2 increases a voltage applied to the current sensing resistor RS as the load current IL rises so that the ratio of the current Imain flowing to the main FET and the current Isense flowing to the sense FET becomes incorrect. Upon a rapid increase in the load current IL, under excessive conditions, the circuit raises the voltage applied to the current sensing resistor RS and thereby lowers the voltages Vgs and Vsense of the sense FET but not the voltages Vgs and Vmain of the main FET, thus inevitably incurring a breakdown of the main FET.
It is an object of the present invention to solve the above problems and to provide an input-separated switching device that has a sense FET so as to control the current accurately and prevent a breakdown of the MOS FET, and a driving circuit of the same.
To achieve the solution to the problems with the prior art, the MOS FET and the MOS FET driving circuit according to the present invention include a main FET and a sense FET, in which the drain of the main FET is connected to the drain of the sense FET and the gate of the main FET is separated from the gate of the sense FET.
In one aspect of the present invention, there is provided an input-separated switching device including: a main switching device for switching a load current portion of the source current based on a first driving signal applied to a first driving signal input; and a sense switching device for switching another portion of the source current based on a second driving signal applied to a second driving signal input in order to check the intensity of the load current.
The main switching device and the sense switching device are a main FET and a sense FET, respectively, wherein a drain of the main FET is connected to a drain of the sense FET, and wherein a gate and a source of the main FET are separated from a gate and a source of the sense FET, respectively.
In another aspect of the present invention, an input-separated switching device driving circuit includes an input-separated switching device, a current sensor, and a current ratio compensator.
The input-separated switching device includes a main switching device for switching a load current portion of the source current based on a first driving signal applied to a first driving signal input, and a sense switching device for switching another portion of the source current based on a second driving signal applied to a second driving signal input in order to check the intensity of the load current.
The current sensor forms a sensing voltage to check the intensity of the load current using current flowing to the sense switching device, the current sensor having one terminal thereof connected to the current output of the sense switching device.
The current ratio compensator applies the sensing voltage to the second driving signal input. The current ratio compensator has a first terminal connected to one terminal of the current sensor, and a second terminal connected to the second driving signal input.
In still another aspect of the present invention, an input-separated switching device driving circuit includes an input-separated switching device, a current sensor, and a current ratio compensator.
The input-separated switching device includes a main switching device for switching a load current portion of the source current based on a first driving signal applied to a first driving signal input, and a sense switching device for switching another portion of the source current based on a second driving signal applied to a second driving signal input in order to check the intensity of the load current.
The current sensor forms a sensing voltage to check intensity of the load current using current flowing to the sense switching device, the current sensor having one terminal thereof connected to a current output of the sense switching device.
The current ratio compensator reduces a voltage applied to the first driving signal input by way of the sensing voltage, the current ratio compensator having a first terminal connected to one terminal of the current sensor, and a second terminal connected to the first driving signal input.
In still another aspect of the present invention, an input-separated switching device driving circuit includes an input-separated switching device, a first delay circuit, a main switch driver, a second delay circuit, a sense switch driver, and a current sensor.
The input-separated switching device includes a main switching device for switching a load current portion of a source current, and a sense switching device for switching another portion of the source current in order to check intensity of the load current.
The first delay circuit receives the second control signal to delay a time, and outputs a delayed second control signal.
The main switch driver switches on the main switching device based on the first control signal, and switches off the main switching device with a delay based on the second control signal output from the first delay circuit.
The second delay circuit receives the first control signal to delay a time, and outputs a delayed first control signal;
The sense switch driver switches on the sense switching device with a delay based on the first control signal output from the second delay circuit, and switches off the sense switching device based on the second control signal.
The current sensor forms a sensing voltage to check the intensity of the load current using current flowing to the sense switching device, the current sensor having one terminal thereof connected to a current output of the sense switching device.
The main switching device and the sense switching device are a main FET and a sense FET, respectively, wherein a drain of the main FET is connected to a drain of the sense FET, and wherein a gate and a source of the main FET are separated from a gate and a source of the sense FET, respectively.